According to the Associated Press, the envelope was intercepted at the U.S. Capitol’s off-site mail processing facility in Prince George’s County, MD, where preliminary tests found the poison. CNNreported the envelope was tested twice more and then sent to another lab in Maryland for further analysis.

Ricin comes from castor beans, and is produced from the 5% to 10% of residue remaining after the oil, used in laxatives and as an automotive lubricant, is extracted. Unintentional exposure to ricin from such processing is highly unlikely, according the CDC, since more steps are required to produce the poison. The toxin can be made as a powder, mist, pellet, weak acid or agent that dissolves in water. Once ingested, either by mouth, inhalation or injection ricin brings a cell’s protein-making machinery to a halt. Without these proteins, the cells to die.

Symptoms of ricin poisoning can appear anywhere from four to 10 hours, with death occurring within 36 to 72 hours following a high enough dose of the toxin.

Inhalation: Within a few hours, the likely symptoms are difficulty breathing, fever, cough, nausea, and tightness in the chest. A victim may start heavily sweating as fluid builds up in their lungs, which causes more breathing difficulty. If low blood pressure and respiratory failure occur, the victim could die.

Ingestion: If significant ricin is swallowed, an exposed victim will likely have vomiting and diarrhea that can have blood. Severe dehydration and low blood pressure can follow. Other symptoms of ingestion may include seizures and blood in the urine. Over several days, a victim’s liver, spleen and kidneys may fail, in which the exposure becomes fatal.

Skin and eye exposure: Ricin is not typically absorbed though the skin, but it could cause redness and pain to the skin and eyes. The bigger concern is if a person touches ricin and then eats with their hands, causing the poison to be ingested.

There is no cure for ricin; the first priority for those exposed is to remove as much of the poison as possible. That includes removing potentially contaminated clothing, washing exposed areas with soap and water, and contacting emergency personnel if someone has ingested the toxin. Medical care is limited to providing help with breathing, intravenous fluids and medications to treat seizures and low blood pressure. To treat immediately exposed victims, physicians may flush stomachs with activated charcoal to absorb the poison.

The letter isn’t the first laced with ricin sent to a Senator. In 2004, a letter containing minute traces of ricin arrived in the mailroom of Senator Bill Frist of Tenn. Prior to that threat, tax-protest groups and truckers opposing new regulations on the number of hours they could spend on the road also tuned to ricin-laden missives mailed to government officials. In 2003, Islamic militants were arrested in Britain for plotting to put ricin in food on a British military base. And most recently, in 2011, four older members of an anti-government militia in Georgia were arrested and accused of planning to attack government structures by releasing ricin from moving cars or an airplane.

But perhaps the most notorious use of ricin occurred as part of an attack worthy of a spy novel. In 1978, Bulgarian journalist Georgi Markov, who had defected to London, was poisoned by a man who injected a ricin pellet under Markov’s skin using an umbrella; Markov died from the attack and while the man was never found, the KGB and Bulgarian secret service were the primary suspects.

In the wake of the most recent mailing, Congressional offices are on alert for additional threats. Sen. Clair McCaskill of Missouri said precautions put in place to screen mail coming into the Capitol “worked.” She told reporters that law enforcement officials had a suspect, who frequently wrote to legislators.